Dirofilaria immitis is a filarial heartworm primarily of canids but which also can infect ferrets and felines. It is a significant cause of heart disease and bronchitis in infected pets. Transmission occurs through feeding by infected mosquitoes, with acute clinical signs occurring through migration of larvae to the pulmonary artery, potentially leading to thromboembolism, caval syndrome and subsequent pulmonary hypertension. Worm death can also lead to anaphylaxis and thromboembolism. Typical resulting acute clinical signs include sudden death, anorexia, weakness, dyspnoea, a jugular pulse and a weak femoral pulse, anaemia, haemoglobinuria, vomiting and rarely pleural effusion. Chronic cardiac and respiratory signs can also develop. Chronic respiratory signs tend to be more common in cats.
Although D. immitis is not endemic in the UK, increasing numbers of infected rescue dogs are being imported from endemic countries. Some of these have already been diagnosed with infection, with new owners in the UK being given varying accounts by rehoming charities regarding how serious infection may be. Others will be unaware that infection is present. It is important that infection is diagnosed quickly so decisions can be made with owners regarding treatment options and prognosis discussed. Although the mosquito vector is endemic in most European countries, including the UK, a colder climate in Northern Europe has prevented heartworm from becoming endemic. This is because the adult mosquito does not live long enough in colder climates for heartworm to complete its life cycle. As a result, there is little chance currently of D. immitis establishing in the UK. Climate change, however, has allowed spread of the parasite northwards, raising the possibility that if this trend continues, endemic foci could establish in the South of England. Veterinary professionals must therefore also be aware of the possibility of infection in dogs imported from endemic countries so UK mosquito vector exposure can be reduced.
A thorough clinical exam of imported pets will identify clinical signs which can then be compared to common exotic parasitic diseases in the countries that the pet has visited. If those signs indicate possible cardiovascular or respiratory disease, then heartworm infection should be considered as a differential. All imported dogs should be screened for heartworm infection however, even if they have already been tested before importation, unless the reporting lab or test can be verified. Testing varies in sensitivity and specificity and the likelihood of infection being missed can only be assessed if the suitability and reliability of the test used can be established. With the exception of serology, testing identifies the presence of adult worms which reach maturity six months post infection. Testing should therefore be repeated six months after arrival in the UK unless the dog has been present in the country for over six months on initial presentation. If the patient is healthy then it is an option to delay testing until six months have elapsed.
Tests to aid in diagnosis include:
- Examination of blood for microfilaria – direct smears are a highly specific test in experienced hands but also insensitive. Concentration techniques such as the modified Knott’s test make direct blood examination more sensitive in canine patients and is useful in helping to quantify numbers of circulating microfilariae and detecting those of other filaroid nematodes such as Dirofilaria repens. If microfilariae are detected, the larvae must be distinguished from D. repens (Table 1) which will also be present in endemic countries. Some UK labs now offer modified Knott’s as a test, eliminating the need to identify microfilariae in practice which can be difficult
- Ultrasound examination – the cuticles of adult heart worms are highly echogenic and so in experienced hands echocardiography can be very sensitive and specific. It can be useful in giving an indication of adult worm burden
- Antigen serology – this is considered the gold standard in the living canine patient. It is highly specific and, in canine patients, also highly sensitive. Sensitivity increases as adult female worm burden increases, with the test detecting antigens in uterine secretions
- Antibody serology – positive antibody serology results only indicate past or present exposure to infection which will be common in dogs living in endemic countries. It is therefore a less appropriate screening test and should be interpreted in relation to clinical signs and other diagnostic tests
Treatment and prevention
Treatment of infections with adult worms requires surgical removal or treatment with an adulticide. Intravascular snares and forceps are often used in endemic countries to remove worms in the least invasive manner.
Echocardiographic visualisation of large numbers of worms in the pulmonary artery allows the use of flexible alligator forceps under fluoroscopic guidance to remove the worms, while avoiding pulmonary thromboembolism. However, these techniques require experience and specialised equipment and as a result most UK vets will require a medical approach to treatment of adult worms.
Before medical treatment is initiated, microfilarial load using a modified Knott’s test should be established as high burdens will increase anaphylaxis risk. The size of adult burden should also be established as large numbers of worms will increase the risk of obstruction and thromboembolism. It is an option not to risk complications of treatment if infected dogs are subclinical and have very low worm burdens. There is a risk of heart disease or thromboembolism developing as worms age and die, however, and the risk of complications from treatment are relatively low if burdens are low and the patient healthy. Risks of fatal complications climb as worm burdens increase if clinical disease is advanced or disease from concurrent infections such as Leishmania infantum or Ehrlichia canis are present. Prognosis and complications from treatment as well as cost and duration should be discussed with clients before it is undertaken.
Dogs with significant clinical signs of heartworm disease should be stabilised before administering an adulticide. This may require administration of glucocorticosteroids, diuretics, vasodilators, positive inotropic agents and fluid therapy.
Adulticide treatment consists of initial treatment with doxycycline and macrocyclic lactones in advance of adulticide treatment. Different protocols are available, but a typical example is:
- Day 1 – doxycycline 10mg/kg sid or bid for 30 days
- Heartworm preventive (macrocyclic lactone) on day 0 and 15
- Day 30 – melarsomine dihydrochloride 2.5mg/kg intramuscular (IM)
- Day 60 and 61 – melarsomine dihydrochloride 2.5mg/kg IM
The patient should then be tested for microfilariae 30 days post treatment and antigen serology tested six months post treatment. The use of doxycycline is to kill Wolbachia bacterial infection residing within the heartworms. The worms and bacteria have an obligatory symbiotic relationship and treatment of the Wolbachia with doxycycline renders the worms more susceptible to treatment (Kramer et al., 2011). There is growing evidence to suggest waiting a month after doxycycline treatment before adulticide is beneficial in eliminating adult worms. If this is to be incorporated into a treatment protocol then a macrocyclic lactone would be administered at day 30 instead of day 15 and adulticides administered at days 60, 90 and 91.
Prednisone has some benefit in reducing the risk of thromboembolic complications if given alongside adulticide treatment where worm burdens are high (Dzimianski et al., 2010). If high worm burdens are suspected, then oral prednisolone can be used from the initiation of adulticide treatment at 0.5mg/kg twice a day for one week and 0.5mg/kg once daily for the second week, followed by 0.5mg/kg every other day for two weeks. These doses are also useful in managing bronchitic signs. There is no evidence that aspirin has any protective effect against thromboembolism in heartworm cases during treatment.
The three most significant factors involved in post adulticide treatment complications are the severity of existing pulmonary vascular disease, the number of worms present and level of exercise. Of these three, over-exercise is thought to be the most significant (Fukami et al., 1998). Exercise should therefore be restricted during treatment, starting from day 0 to at least one month after the last adulticide injection. This will reduce the risk of severe thromboembolism. Cardiovascular disease can be assessed by radiography and ultrasonography and adult burden is crudely estimated by ultrasonography. Elimination of adult worms remains a priority however, even in high-risk patients although in the non-acute patient there is the opportunity to attempt to stabilise cardiac and pulmonary signs with supportive treatment before adulticide treatment is initiated. The highest risk period of complications from pulmonary thromboembolism is 7 to 10 days after adulticide treatment but can occur up to four weeks after adulticide treatment is completed.
An alternative treatment which avoids the use of the adulticide melarsomine is the “slow kill” regime. This should not be used as a first-choice treatment as it has been linked to the development of resistance (Bowman et al., 2012), and carries some risk of anaphylaxis due to the killing of microfilariae from active infection over a long period of time. This is particularly true when large numbers of microfilariae are present in the circulation. There is also no decreased risk of thromboembolism using this method. It should therefore only be used if surgery is not indicated or practical and melarsomine is not available or unaffordable.
The protocol is:
- Doxycycline 10 to 20mg/kg SID or BID for 30 days
- Ivermectin at minimum dosage of 6 to 12mg/kg monthly or topical moxidectin 2.5mg/kg monthly until two consecutive negative antigen test results after 12 months
Exercise restriction is also recommended with this protocol from the start of treatment until infection is eliminated.
Heartworm is a serious and potentially fatal infection of dogs, and owners adopting infected dogs need to be aware of the risks of ongoing infection and treatment options. Many owners will adopt new pets from endemic countries with no idea that they are infected and rapid diagnosis and recognition of relevant clinical signs is essential both for welfare of the individual dog and to minimise the risk of UK mosquito infections being exposed to the parasite in the future. While there is a risk of fatal complications from both infection and treatment, many dogs will go on to make a full recovery if risk factors for complications are carefully managed, worm burdens low and clinical disease not advanced.